Ok, so basically trisomy 21 is another name for Down's syndrome. As you've already said, human's normally have 46 chromosomes, or twenty-three pairs, which they are often divided into, because each pair looks similar. I'll include a picture. Anyways, normally each pair has 2, but in the event that there is a mutation, then there can be a monomy or a trisomy. A monomy is where there is only one of the two chromosomes in the pair, or a trisomy is an extra chromosome. So that's about it, hope the background info helps. :D